Because of the extreme nature of the application, materials used for fabrication of ammunition cartridges must demonstrate excellent mechanical and thermal properties. As such, the prevalent materials for production of cartridge cases for all calibers of ammunition in the world today are metals. Brass is the leading material, followed in smaller amounts by steel and, in limited amounts, aluminum. Brass, steel, and, to a lesser degree, aluminum cartridge cases suffer from a number of disadvantages, the most important of which are their heavy weight and susceptibility to corrosion. Aluminum has the added disadvantage of potentially explosive oxidative degradation, and is thus used only in low-pressure cartridges or in applications that can tolerate relatively thick casing walls.
Given these issues, desirable materials for ammunition cartridge casing fabrication would be lightweight and impervious to corrosion while having mechanical properties suitable for use in ammunition applications. Many lightweight polymeric materials are sufficiently corrosion resistant; however, to date, polymers have been used only in niche ammunition applications where their inferior mechanical and thermal properties can be tolerated (e.g., shotgun shells, which often contain polyethylene components). While the use of polymeric materials for ammunition cartridge cases has been extensively investigated over the past 40 years, but success has been elusive. Recently new types of polymeric materials have been identified that address many of the mechanical and thermal deficiencies of previous polymeric materials. (See, e.g., U.S. Patent Pub. No. 2006-0207464, the disclosure of which is incorporated herein by reference.)
While progress has been made on possible polymeric materials for use in forming ammunition cartridge casings, a number of engineering challenges remain in adapting conventional ammunition cartridge casing designs for use with these new materials. In particular, weatherability and stability under broad ranges of handling and storage conditions are important, but the greatest mechanical demands on the cartridge are experienced during the firing event. The material at the cartridge base end, which supports the primer, must first absorb the impact of a firing pin on the primer without mechanical failure. Upon ignition and combustion of an encapsulated propellant, rapidly expanding gases create high pressure, which expels a projectile from the barrel of the fired weapon. The ammunition cartridge casing must withstand and contain the pressure developed by the explosion so that the gaseous combustion products expand only in the direction of the barrel opening, thus maximizing energy conversion to projectile kinetic energy.
A weapon's cartridge chamber supports the majority of the cartridge casing wall in the radial direction, but, in many weapons, a portion of the cartridge base end is unsupported. During firing, a stress profile is developed along the cartridge casing, the greatest stresses being concentrated at the base end. Therefore, the cartridge base end must possess the greatest mechanical strength, while a gradual decrease in material strength is acceptable in brass cartridges axially along the casing toward the end that receives the projectile. This is especially important in case of repeating weapons such as machine guns and assault rifles. Often, the cartridges being extracted out of repeating weapons will still contain combustion gas pressure and the round has to be able to withstand extraction event while still being partially pressurized. For reference, typical peak chamber pressures in modern rifles and machine guns are between 35,000 and 70,000 psi. Depending on the cycle time of the individual repeating weapons, the pressure at extraction will vary between 0% and 50% of the peak chamber pressure.
Accordingly, a need exists to develop ammunition cartridge casing geometries optimized for use with modern polymeric materials.